Ocean Quahog (Mahogany Clam, Black Clam)
Kingdom: Animalia, Phylum: Mollusca, Class: Bivalvia, Order: Carditida, Family: Arcticidae, Genus: Arctica, Species: Arctica islandica · Arcticidae (Iceland Cyprine family) · Bivalve (Clam); Sub-circular/Oval thick shell
Species
Arctica islandica
Shell Type
Bivalve (Clam); Sub-circular/Oval thick shell
Family
Arcticidae (Iceland Cyprine family)
Size
3.5 to 4 inches (8.9 to 10.2 cm), which is typical for a mature adult. They can reach up to 5 inches.
Color & Pattern
The exterior is covered in a thick, dark mahogany-brown to blackish periostracum. In older specimens like this one, it appears nearly black and somewhat flaky. Where the periostracum is worn away at the umbo (beak), the underlying shell color is typically chalky white or cream.
Rarity
Common (Beachcombing finds are frequent after storms, though live specimens reside in deeper water).
Habitat
Found in cold temperate to boreal waters, typically burrowed in sandy or muddy sediments. Depth ranges from the low intertidal zone down to 400 meters (1,300 feet).
Geographic Range
North Atlantic Ocean; found along the East Coast of North America from Newfoundland to Cape Hatteras, and in Europe from the Arctic Ocean to the Bay of Biscay.
Description
A heavy, thick-walled bivalve known for its extraordinary longevity. The shell is roughly circular to oval with a prominent, forward-curving umbo. The surface is marked by fine concentric growth lines that become more crowded towards the margins. This specimen shows a well-preserved, dark periostracum that gives it its characteristic 'mahogany' appearance.
Key Features
The heavy weight relative to size; dark, thick periostracum; lack of a pallial sinus (an internal marking); and the absence of radiating ribs distinguish it from other large clams.
Collector Value
Low to Moderate. While common, they are prized by enthusiasts of biological longevity. Exceptional pairs with perfectly preserved periostracums may fetch $10-$20 in the collector market.
Condition Notes
Good condition. The periostracum is substantially intact, which is desirable for showing its natural state, though it shows some expected weathering and minor chipping at the edges. Collectibility grade: Fine.
Interesting Facts
The Ocean Quahog is one of the longest-lived animals on Earth. One specimen, nicknamed 'Ming the Mollusk,' was found to be 507 years old. Their age can be determined by counting shell growth rings, similar to tree rings.
Ecological Role
A long-lived filter feeder that helps cycle nutrients by consuming phytoplankton. They serve as a food source for cod, crustaceans, and sea stars, and are an important indicator species for climate change research.
Similar Species
Mercenaria mercenaria (Hard Clam/Quahog), which has a more triangular shape, purple interior staining, and lacks the thick dark periostracum. Also similar to Glycymeris (Bittersweet) shells, which have distinct radial hinge teeth.
Beach Finding Tips
Look for these on Atlantic beaches after strong northeasters or winter storms which wash them up from deeper offshore beds. They are often found partially buried in the wrack line.